Reduce log spam for the ease of debugging Bug 22857361.

Now InputMethodManagerService generates the following log
    Couldn't create dir.: /data/system/inputmethod
not only when it fails to create the directory but also when
/data/system/inputmethod already exists, which makes it
difficult for us to figure out the root cause of boot failure
on emulator environments (Bug 22857361).

With this CL, IMMS no longer shows the message when the
directory already exists.  Basically this is no risk change,
which changes only the condition to show the logging message.

Bug: 22857361
Change-Id: I09aaf501b19845c8309b09b57c23077f1757cd1a